One type of heating ventilation and air conditioning (HVAC&R) system uses a chilled fluid to remove heat from a building and is typically referred to as a chilled water system. The fluid utilized in the chilled water system is not limited to water and may include liquids, such as glycol or brine. In this type of system, a chilled fluid is provided to a building having a heating load. The chilled fluid is placed in a heat exchange relationship with the heating load from the building, usually warm air. During the heat exchange with the heating load, the chilled fluid receives heat from the heating load and generally increases in temperature. In order to remove the heat from the fluid and to lower the temperature of the fluid, a closed loop refrigeration system is utilized. The fluid circulated through the building is chilled by placing the fluid in a heat exchange relationship with another cooler fluid, usually a refrigerant, in a heat exchanger, commonly referred to as an evaporator or chiller. The refrigerant in the evaporator removes heat from the fluid during the evaporation process, thereby cooling the fluid. The chilled fluid is then circulated back to the building for subsequent heat exchanging with the heating load, and the cycle repeats.
Chillers may include a shell and tube heat exchanger design. The shell and tube heat exchanger may include a bundle of heat exchange tubes located in a shell. The tubes are typically fabricated from a metal, such as copper, and may be horizontally mounted. At either end of the tubes are tube sheets that support the individual tubes. Refrigerant may flow through the tubes in order to cool a fluid, usually water or an aqueous solution, flowing through the shell. The use of this type of shell and tube heat exchanger design in an evaporator is commonly referred to as a direct expansion (DX) evaporator. A typical design for DX evaporators includes a single inlet connection and a single outlet connection for the fluid flowing through the shell. The single inlet and single outlet provide a single flow stream of fluid that exchanges heat with the refrigerant flowing inside the tubes. The shell side flow of the fluid follows a serpentine path due to the use of a plurality of baffles inside the shell on the shell side. The shell side fluid flow is generally in one direction providing uneven heat exchange over the length of the shell. Furthermore, DX evaporators incorporating tubes for multiple refrigerant circuits must flow the refrigerant in a single, concurrent direction. For a given shell side fluid flow, the DX evaporators effectiveness depends upon the direction of the refrigerant flow. Known evaporators provide efficient operation and superheated refrigerant by exchanging heat between the outlet flow of refrigerant and the inlet flow of fluid, i.e., by having the shell side fluid flow be opposite the refrigerant flow. The inlet flow of fluid contains an amount of heat greater than the outlet flow of fluid. Therefore, in order to operate efficiently, known DX evaporators must flow the shell side fluid in a single direction in order to efficiently provide heat to the refrigerant outlet.
The refrigerant in the tubes may make multiple passes across the shell through the use of baffling in the headers of the evaporator. However, known DX evaporators utilized in chilled water systems suffer from the drawback that the diameter of the shell of the evaporator becomes relatively large as the total heat exchange capacity increases and the shell requires a larger vertical clearance (i.e., heat exchanger height) in which to install. In particular, known DX evaporators having multiple passes require a large vertical clearance in the chiller platform providing for increased difficulty in installation. In addition, water flowing into the shell through the single inlet could cause excessive tube vibration, which could eventually cause failure of the tubes due to fatigue.
What is needed is an evaporator that permits refrigerant flow in either direction through the tubes, has a relatively small shell diameter, and a reduced tube vibration.